Blood cells originate from hematopoietic stem cells that become committed to differentiate along certain lineages, i.e., erythroid, megakaryocytic, granulocytic, monocytic, and lymphocytic. Cytokines that stimulate the proliferation and maturation of cell precursors are called colony stimulating factors (“CSFs”). Several CSFs are produced by T-lymphocytes, including interleukin-3 (“IL-3”), granulocyte-monocyte CSF (GM-CSF), granulocyte CSF (G-CSF), and monocyte CSF (M-CSF). These CSFs affect both mature cells and stem cells. Heretofore no factors have been discovered that are able to predominantly affect stem cells.
Tyrosine kinase receptors (“TKRs”) are growth factor receptors that regulate the proliferation and differentiation of a number of cells (Yarden, Y. & Ullrich, A. Annu. Rev. Biochem., 57, 443–478, 1988; and Cadena, D. L. & Gill, G. N. FASEB J., 6, 2332–2337, 1992). Certain TKRs function within the hematopoietic system. For example, signaling through the colony-stimulating factor type 1 (“CSF-1”), receptor c-fms regulates the survival, growth and differentiation of monocytes (Stanley et al., J. Cell Biochem., 21, 151–159, 1983). Steel factor (“SF”, also known as mast cell growth factor, stem cell factor or kit ligand), acting through c-kit, stimulates the proliferation of cells in both myeloid and lymphoid compartments.
Flt3 (Rosnet et al. Oncogene, 6, 1641–1650, 1991) and flk-2 (Matthews et al., Cell, 65, 1143–1152, 1991) are variant forms of a TKR that is related to the c-fms and c-kit receptors. The flk-2 gene product is expressed on hematopoietic and progenitor cells, while the flt3 gene product has a more general tissue distribution. The flt3 and flk-2 receptor proteins are similar in amino acid sequence and vary at two amino acid residues in the extracellular domain and diverge in a 31 amino acid segment located near the C-termini (Lyman et al., Oncogene, 8, 815–822, 1993).
Flt3-ligand (“flt3-L”) has been found to regulate the growth and differentiation of progenitor and stem cells and is likely to possess clinical utility in treating hematopoietic disorders, in particular, aplastic anemia and myelodysplastic syndromes. Additionally, flt3-L will be useful in allogeneic, syngeneic or autologous bone marrow transplants in patients undergoing cytoreductive therapies, as well as cell expansion. Flt3-L will also be useful in gene therapy and progenitor and stem cell mobilization systems.
Cancer is treated with cytoreductive therapies that involve administration of ionizing radiation or chemical toxins that kill rapidly dividing cells. Side effects typically result from cytotoxic effects upon normal cells and can limit the use of cytoreductive therapies. A frequent side effect is myelosuppression, or damage to bone marrow cells that give rise to white and red blood cells and platelets. As a result of myelosuppression, patients develop cytopenia, or blood cell deficits, that increase risk of infection and bleeding disorders.
Cytopenias increase morbidity, mortality, and lead to under-dosing in cancer treatment. Many clinical investigators have manipulated cytoreductive therapy dosing regimens and schedules to increase dosing for cancer therapy, while limiting damage to bone marrow. One approach involves bone marrow or peripheral blood cell transplants in which bone marrow or circulating hematopoietic progenitor or stem cells are removed before cytoreductive therapy and then reinfused following therapy to restore hematopoietic function. U.S. Pat. No. 5,199,942, incorporated herein by reference, describes a method for using GM-CSF, IL-3, SF, GM-CSF/IL-3 fusion proteins, erythropoietin (“EPO”) and combinations thereof in autologous transplantation regimens.
High-dose chemotherapy is therapeutically beneficial because it can produce an increased frequency of objective response in patients with metastatic cancers, particularly breast cancer, when compared to standard dose therapy. This can result in extended disease-free remission for some even poor-prognosis patients. Nevertheless, high-dose chemotherapy is toxic and many resulting clinical complications are related to infections, bleeding disorders and other effects associated with prolonged periods of myelosuppression.
Myelodysplastic syndromes are acquired stem cell disorders characterized by impaired cellular maturation, progressive pancytopenia, and functional abnormalities of mature cells. They have also been characterized by variable degrees of cytopenia, ineffective erythropoiesis and myelopoiesis with bone marrow cells that are normal or increased in number and that have peculiar morphology. Bennett et. al. (Br. J. Haematol. 1982; 51:189–199) divided these disorders into five subtypes: refractory anemia, refractory anemia with ringed sideroblasts, refractory anemia with excess blasts, refractory anemia with excess blasts in transformation, and chronic myelomonocytic leukemia. Although a significant percentage of these patients develop acute leukemia, a majority die from infectious or hemorrhagic complications. Treatment of theses syndromes with retinoids, vitamin D, and cytarabine has not been successful. Most of the patients suffering from these syndromes are elderly and are not suitable candidates for bone marrow transplantation or aggressive antileukemic chemotherapy.
Aplastic anemia is another acquired disease entity that is characterized by bone marrow failure and severe pancytopenia. Unlike myelodysplastic syndrome, the bone marrow is acellular or hypocellular in this disorder. Current treatments include bone marrow transplantation from a histocompatible donor or immunosuppressive treatment with antithymocyte globulin (ATG). Similarly to myelodysplastic syndrome, most patients suffering from this syndrome are elderly and are unsuitable for bone marrow transplantation or for aggressive antileukemic chemotherapy. Mortality in these patients is exceedingly high from infectious or hemorrhagic complications.
Another hematopoietic disorder involving multiple blood lineages includes Fanconi's anemia (a congenital deficiency of red cells, white cells and platelets). Plycythenias are diseases characterized by the overproduction of red cells, white cells and platelets.